Author Topic: The effect of Wind  (Read 1853 times)

quixoticgeek

  • Mostly Harmless
Re: The effect of Wind
« Reply #25 on: March 13, 2019, 02:18:19 pm »
I never got the PowerPod to work. Expensive paper weight.

Mind if I swap it for another heavy object and see if I can make it work?

Round these parts, slogging into a headwind is basically par for the course, On my most recent headwind training session (Yes I am crazy enough to seek out a strong headwind for training!), a paired strava segment (2 segments for same stretch of road, but in each direction), it took me 20:54 in one direction, and 8:06 in the reverse, that was with winds 45+kph. The segment was 4.9km.

No tailwind will make up for the headwind you suffered to get there...

J
--
Beer, bikes, and backpacking
http://b.42q.eu/

Re: The effect of Wind
« Reply #26 on: March 13, 2019, 02:30:34 pm »
Can a mechanics nerd explain to me why the power required to ride at 15 mph into a 15 mph headwind is much less than that needed to ride at 30 mph on a still day (ignoring non-aero forces)?

Surely from the reference frame of either the air or the bike the work being done is the same - why is the relative position of the earth relevant?

It seems the standard equation is this:

P = C * S * (S+H)^2

(Where S is ground speed, H is headwind and C is an aero drag constant)

Why is that not C * (S+H)^3? I'm sure the equation above is correct, I just can't get my head around *why* it is correct - what less work is being done?

mattc

  • n.b. have grown beard since photo taken
    • Didcot Audaxes
Re: The effect of Wind
« Reply #27 on: March 13, 2019, 04:10:49 pm »
It's impossible to answer "why" questions - Why is Gravity?? - but we can probably help the keen student reach his/her own level of satisfaction :P

Two hints:
- what is the definition of "Work done"?
- Look at your equations, and consider the case of S=0.
Has never ridden RAAM
---------
No.11  Because of the great host of those who dislike the least appearance of "swank " when they travel the roads and lanes. - From Kuklos' 39 Articles

Re: The effect of Wind
« Reply #28 on: March 14, 2019, 06:15:13 pm »
Can a mechanics nerd explain to me why the power required to ride at 15 mph into a 15 mph headwind is much less than that needed to ride at 30 mph on a still day (ignoring non-aero forces)?

Surely from the reference frame of either the air or the bike the work being done is the same - why is the relative position of the earth relevant?

It seems the standard equation is this:

P = C * S * (S+H)^2

(Where S is ground speed, H is headwind and C is an aero drag constant)

Why is that not C * (S+H)^3? I'm sure the equation above is correct, I just can't get my head around *why* it is correct - what less work is being done?
The effort required to overcome rolling resistance rises as a linear function of speed - riding at double the speed results in a considerable increase in output overcoming rolling resistance.
<i>Marmite slave</i>

Re: The effect of Wind
« Reply #29 on: March 14, 2019, 10:37:34 pm »
- Look at your equations, and consider the case of S=0.

Having pondered this, I think the explanation I’m looking for is that in the headwind example, the wind is doing part of the work of moving the air past the rider, hence the rider needs to do less work than when they’re forcing their way through static air on the own.

Quote from: mrcharly-YHT link=topic=111352.msg2377946#msg2377946 date=
effort required to overcome rolling resistance rises as a linear function of speed - riding at double the speed results in a considerable increase in output overcoming rolling resistance.

I’m looking at the equation for aero drag in isolation.

simonp

  • Omnomnomnipotent.
Re: The effect of Wind
« Reply #30 on: March 14, 2019, 10:45:11 pm »
It's really very simple.

In moving an object against a force, work done = force * distance travelled. Power is work per second, therefore it's force * speed.

In the case of 30mph in still air vs 15mph into 15mph wind, the force is the same in both cases, but the speed is halved in the 15mph case. Hence it requires half as much power.

Rolling resistance also comes into it, but is a much smaller factor at these kind of speeds.


Re: The effect of Wind
« Reply #31 on: March 16, 2019, 10:24:11 pm »
Please also note that wind speed is usually measured 10 meters above the ground. Wind speed drops significantly as you approach the ground with speed being 0 at the ground (boundary layers). As a rough approximation, wind speed is 1/2 that 1m above the ground than it is at 10m and 1m is where you, the dragiest part of the system reside. Essentially, 15mph quoted does not equal 15mph across your whole bike, if any of it.

Furthermore, it's rare that the wind hits you directly from the front. I'm slowly working my way through discussions from a chap called Hambini who reckons that yaw is much greater than previously suspected as he takes transient airflow into account instead of steady state that most people use.

With all this info, the equation of choice is D = Cd * A * 0.5 * r * V^2. Assuming air density, frontal area and coefficient of drag all stay the same, and if we assume that the wind is only half that measured at 10m (ie. 7.5mph), total velocity is increased by 50% to give a drag increase of 2.25x vs 4x at 30mph. Drag is directly proportional to required power output

So in summary, riding at 15mph into a "15mph" headwind will approximate to riding at 22.5mph.

*there's talk that a still day isn't a still day though, but let's not get into that....

Kim

  • Timelord
Re: The effect of Wind
« Reply #32 on: March 17, 2019, 12:35:54 am »
Please also note that wind speed is usually measured 10 meters above the ground. Wind speed drops significantly as you approach the ground with speed being 0 at the ground (boundary layers). As a rough approximation, wind speed is 1/2 that 1m above the ground that it is at 10m and 1m is where you, the dragiest part of the system reside.

The boundary effect makes a real difference to a low recumbent in windy conditions, even if it's not a particularly aerodynamic one (eg. tadpole trikes at the more touring end of the spectrum).  Harder to quantify theoretically is the effect of being low enough to get meaningful shelter from even minimal roadside vegetation.  Anecdotally, that makes a huge difference to crosswinds (as you get reminded every time you pass a field entrance).
To ride the Windcheetah, first, you must embrace the cantilever...

Re: The effect of Wind
« Reply #33 on: March 18, 2019, 11:17:55 am »
Please also note that wind speed is usually measured 10 meters above the ground. Wind speed drops significantly as you approach the ground with speed being 0 at the ground (boundary layers). As a rough approximation, wind speed is 1/2 that 1m above the ground that it is at 10m and 1m is where you, the dragiest part of the system reside.

The boundary effect makes a real difference to a low recumbent in windy conditions, even if it's not a particularly aerodynamic one (eg. tadpole trikes at the more touring end of the spectrum).  Harder to quantify theoretically is the effect of being low enough to get meaningful shelter from even minimal roadside vegetation.  Anecdotally, that makes a huge difference to crosswinds (as you get reminded every time you pass a field entrance).

I always think this makes sense, but am surprised that TT bikes don't tend to come with lower bottom brackets, and that riders don't seek out the lowest pedal system stack heights, to get themselves lower to the ground.  Given the obsessive focus of testers on buying any speed going, anyone know why this isn't more of a thing? 
Maybe I'll brave going back onto the TT forum and see what responses I get...

Zed43

  • prefers UK hills over Dutch mountains
Re: The effect of Wind
« Reply #34 on: March 18, 2019, 01:03:00 pm »
Maybe they're worried about pedals striking the ground when cornering (I tend not to pedal through corners, but then I am not a racer). Or maybe the UCI has rules on the bottom bracket drop like they have on the position of the saddle?

Kim

  • Timelord
Re: The effect of Wind
« Reply #35 on: March 18, 2019, 01:18:16 pm »
There must be a UCI rule in there somewhere, otherwise people would use low-racers.
To ride the Windcheetah, first, you must embrace the cantilever...

Re: The effect of Wind
« Reply #36 on: March 18, 2019, 02:35:51 pm »
There are UCI rules about where the saddle can be wrt the BB, and where the aero extensions can be wrt the saddle and the head tube, so you end up with a bike that looks a lot like a bike and not a low-racer.
Whether there are any rules about dropping the BB down I don't know. There's only so far you can go though while retaining decent leg extension and avoiding pedal strike. I know at least one of the TTF crowd has experimented with reducing leg extension to get everything lower, but found the power loss too difficult to overcome.

Re: The effect of Wind
« Reply #37 on: March 18, 2019, 02:56:19 pm »
BB spindle must be 24-30 cm above the ground:
https://www.uci.org/docs/default-source/equipment/approval-protocol-eng.pdf?sfvrsn=c07e81c6_12

Given a wheel radius of 334 mm (700x23), that's a BB drop of 34-94mm. A quick google suggests the lowest TT BBs are 80mm drop, and most are 60-65mm, so it's probably not UCI rules that are limiting it. I guess you may run into other geometry limitations - head tube length, downtube clearance, etc as well as the pedal strike risk.

Re: The effect of Wind
« Reply #38 on: March 18, 2019, 08:46:52 pm »
There must be a UCI rule in there somewhere, otherwise people would use low-racers.

It's CTT, not UCI.  There isn't a rule on BB height 
https://www.cyclingtimetrials.org.uk/articles/view/11

Re: The effect of Wind
« Reply #39 on: March 18, 2019, 08:49:51 pm »
Maybe they're worried about pedals striking the ground when cornering (I tend not to pedal through corners, but then I am not a racer). Or maybe the UCI has rules on the bottom bracket drop like they have on the position of the saddle?

People take greater risks than a pedal strike on a roundabout in pursuit of speed.  Most riders would trade-off a little bit of speed on roundabouts to go faster between them!

Karla

  • car(e) free
    • Lost Byway - a Pacific bike ride
Re: The effect of Wind
« Reply #40 on: March 18, 2019, 09:21:00 pm »
People do indeed seek out low stack pedals.  They also experiment with reduced leg extension, and of you look at TT pics from the last few years you'll see that a good proportion of riders have switched to running their saddles very far back so they get as low as possible while maintaining leg extension, albeit with a tighter hip angle.  Basically they're trying to approximate a recumbent within the bicycle rules

Re: The effect of Wind
« Reply #41 on: March 19, 2019, 07:31:25 am »
People do indeed seek out low stack pedals.  They also experiment with reduced leg extension, and of you look at TT pics from the last few years you'll see that a good proportion of riders have switched to running their saddles very far back so they get as low as possible while maintaining leg extension, albeit with a tighter hip angle.  Basically they're trying to approximate a recumbent within the bicycle rules

Yes, that happens but nobody seems to go on about low BB, and not many change pedals for lower stack.

CrazyEnglishTriathlete

  • Miles eaten don't satisfy hunger
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Re: The effect of Wind
« Reply #42 on: March 19, 2019, 09:10:12 am »
A headwind is good for the soul.  And bugger all use for anything else.
Eddington Numbers 124 (imperial), 168 (metric) 517 (furlongs)  111 (nautical miles)

quixoticgeek

  • Mostly Harmless
Re: The effect of Wind
« Reply #43 on: March 19, 2019, 01:02:44 pm »
A headwind is good for the soul.  And bugger all use for anything else.

Hill training, when you have no hills...

J
--
Beer, bikes, and backpacking
http://b.42q.eu/

CrazyEnglishTriathlete

  • Miles eaten don't satisfy hunger
  • 3x Brimstone ancien 3x Pendle/Tan Hill DNF
    • CET Ride Reports and Blogs
Re: The effect of Wind
« Reply #44 on: March 19, 2019, 02:33:45 pm »
A headwind is good for the soul.  And bugger all use for anything else.

Hill training, when you have no hills...

J

That fits under the good for the soul category.  If you get saintly enough you can float up the hills  :facepalm:
Eddington Numbers 124 (imperial), 168 (metric) 517 (furlongs)  111 (nautical miles)